1. Technical Field of the Invention
The present invention relates generally to an apparatus and a method for adjusting or defining a desired spark gap of spark plugs precisely at high speeds.
2. Background Art
Typical spark plugs consist of a center electrode, a metal housing or shell retaining the periphery of the center electrode through a porcelain insulator, and a ground electrode secured in the metal shell so as to face the center electrode. The interval between the center electrode and the ground electrode is adjusted to, for example, 1 mm to define a suitable spark gap.
Japanese Patent No. 2636814 discloses a spark gap adjustment method. This method includes steps of holding a spark plug, capturing an image of the spark plug around ends of a center electrode and a ground electrode through a CCD camera to produce an image signal, processing the image signal to measure the interval between the ground electrode and the center electrode, and pressing an outer wall of the ground electrode toward the center electrode using a press unit to adjust the ground-to-center electrode interval to a desired value.
In order to improve the productivity of spark plugs, quick adjustment of the spark gap is sought. The above method, however, performs the step of pressing the ground electrode while measuring the ground-to-center electrode interval and thus encounters a problem that the speed at which the press unit is moved to push the ground electrode depends upon the time required to measure the ground-to-center electrode interval, and too fast pushing the ground electrode will make it difficult to measure the ground-to-center electrode interval precisely.
For instance, if the time required to scan one frame of an image captured by the CCD camera is 0.2 sec., a typical tolerance of a 1 mm spark gap is 0.1 mm, therefore, an allowable maximum speed of the press unit will be 0.5 mm/sec. Increasing the accuracy of measuring the ground-to-center electrode interval requires decreasing the speed of the press unit.